Spinal Interbody Devices with Density Gradients and Associated Methods

ABSTRACT

An interbody device configured for insertion between adjacent vertebrae includes a body comprising and exterior surface and an interior surface defining a cavity. The body comprises a visualization window extending between the exterior surface and the interior surface, where the visualization window comprises a lattice of radiopaque structures. A density of the lattice in a central region of the visualization window is less than in the density of the lattice in an outer region of the visualization window such that the visualization window is radiolucent through the central region.

TECHNICAL FIELD

The present disclosure relates generally to a composite interbody deviceadapted for insertion between two adjacent vertebrae to promote thefusion of two vertebrae.

BACKGROUND

The bones and connective tissue of an adult human spinal column consistsof more than 20 discrete bones coupled sequentially to one another by atri-joint complex. The complex consists of an anterior disc and twoposterior facet joints. The anterior discs of adjacent bones arecushioned by cartilage spacers referred to as intervertebral discs. Theover 20 bones of the spinal column are anatomically categorized as oneof four classifications: cervical, thoracic, lumbar, or sacral. Thecervical portion of the spine which comprises the top of the spine up tothe base of the skull, includes the first 7 vertebrae. The intermediate12 bones are thoracic vertebrae, and connect to the lower spinecomprising the 5 lumbar vertebrae. The base of the spine are sacralbones, including the coccyx.

The spinal column of bones is highly complex in that it includes over 20bones coupled to one another, housing and protecting critical elementsof the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity Despite its complexity, the spineis a highly flexible structure, capable of a high degree of curvatureand twist in nearly every direction.

Genetic or developmental irregularities, trauma, chronic stress, tumorsand disease, however, can result in spinal pathologies which eitherlimit this range of motion or threaten the critical elements of thenervous system housed within the spinal column. A variety of systemshave been disclosed in the art which achieve immobilization byimplanting artificial assemblies in or on the spinal column. Theseassemblies may be classified as anterior, posterior or lateral implants.Lateral and anterior assemblies are coupled to the anterior portion ofthe spine which is in the sequence of vertebral bodies. Posteriorimplants generally comprise pairs of rods (“bilateral spinal supportrods”), which are aligned along the axis which the bones are to bedisposed, and which are then attached to the spinal column by eitherhooks which couple to the lamina or attach to the transverse processes,or by screws which are inserted through pedicles.

Spinal fusion treatment is commonly used to treat spinal disc diseaseand/or spinal instability. The degeneration of spinal discs can createsignificant pain and discomfort for individuals suffering from thisaffliction. In many cases, this pain can be alleviated by immobilizingthe vertebrae adjacent to the degenerated disc and encouraging bonegrowth across the immobilized area of the spine. Conventional spinalimplants are designed to facilitate bone through-growth, or fusionresulting from growth of bone through holes or channels through theimplants. Although effective, the bone through-growth process is slow,sometimes taking more than a year to complete. Through-growth can befurther delayed if the implant area is not immobilized. Evenmicro-motion of the implant area can disturb and disrupt bone growth,leading to increased incidence of subsidence and pseudarthrosis.

Some conventional devices attempt to improve implant stabilization byencouraging bone on-growth—a comparatively rapid, planar growth of boneupon surfaces of an adjacent implant, or upon surfaces of adjacent bone.For example, on-growth may be encouraged by coating a titanium cage witha chemical such as hydroxyapatite to encourage new-grown bone to adhereto the implant surface. However, because titanium is radiopaque,titanium implants can interfere with diagnostic assessment of bonegrowth, whether coated with hydroxyapatite or not. For example, titaniumimplants may obscure visualization of bone growth (e.g., through-growth)on x-rays, making it difficult to determine if fusion has occurred.

SUMMARY

Various embodiments of an interbody device for use with spinal fusionsurgery are described herein. The interbody device may include bodyhaving apertured or porous visualization windows, and solid scaffoldportions. The visualization windows may have a porosity gradient ordensity gradient along at least one axis of the implant. In someembodiments, the density decreases toward the center of thevisualization windows to provide for increased radiotransparency. Thevisualization windows may have higher densities near the outer edges andouter areas of the visualization windows to maintain strength andstructural integrity under the load of the spine. The density may varyas a gradient, which may be linear or nonlinear. The solid scaffoldportions provide additional strength and reinforcement. Accordingly, theimplants provided herein are sufficiently strong for use as interbodydevices in spinal fusion surgery, while allowing for x-ray imagingthrough the visualization windows for monitoring the progress of bonein-growth into the porous structures of the implants.

According to one embodiment, an interbody device configured forinsertion between adjacent vertebrae includes a body comprising andexterior surface and an interior surface defining a cavity. The bodycomprises a visualization window extending between the exterior surfaceand the interior surface, where the visualization window comprises alattice of radiopaque structures. A density of the lattice in a centralregion of the visualization window is less than in the density of thelattice in an outer region of the visualization window such that thevisualization window is radiolucent through the central region.

In some aspects, the body comprises a top side and a bottom side, andwherein the lattice comprises a first density gradient along a firstdirection extending between the top side and the bottom side. In someaspects, the lattice comprises a second density gradient along a seconddirection transverse to the first direction. In some aspects, the firstdensity gradient linearly decreases from the outer region of thevisualization window to the central region of the visualization window.In some aspects, the density of the lattice of the outer region of thevisualization window is associated with a porosity of 40%-60%, andwherein the density of the lattice of the central region of thevisualization window is associated with a porosity of 70%-90%.

In some aspects, the visualization window comprises a first constantdensity region in the outer region. In some aspects, the first constantdensity region extends from a bottom edge of the body to a firstintermediate region between the bottom edge and the central region ofthe visualization window. In some aspects, the lattice of radiopaquestructures comprises a gyroid architecture. In some aspects, the densityof the lattice is based on geometric parameters of the gyroidarchitecture.

In some aspects, the body comprises a plurality of solid scaffoldregions, wherein the visualization window is disposed between two solidscaffold regions. In some aspects, the body comprises a sidewallextending from the exterior surface to the interior surface, and whereinthe plurality of solid scaffold regions occupy only a portion of athickness of the sidewall. In some aspects, the device further includesa solid wall on a lateral side of the bod. In some aspects, the bodydefines one or more passages extending through the solid wall on thelateral side of the body. In some aspects, the body comprises a sidewallextending from the exterior surface to the interior surface, and whereinthe lattice of the visualization window occupies an entire thickness ofthe sidewall. In some embodiments, the lattice of radiopaque structurescomprises a metal. In some embodiments, the lattice of radiopaquestructures comprises titanium.

According to another embodiment of the present disclosure, anintervertebral spacer configured to be placed between two adjacentvertebrae includes: a solid portion comprising a plurality of solidscaffold regions, wherein the solid scaffold regions are disposed aroundan opening; and a visualization window disposed between two or more ofthe solid scaffold regions and around the opening. The visualizationwindow comprises a lattice of radiopaque surfaces defining a network ofpores. A density of the lattice is greater in an outer region of thevisualization window than in a central region of the visualizationwindow such that the visualization window is radiolucent in the centralregion.

In some aspects, the solid portion and the visualization window form aunitary body. In some embodiments, the lattice varies linearly indensity from the outer region to a center of the central region along avertical axis. In some embodiments, the visualization window furthercomprises a constant density region positioned above the outer regionsuch that the outer region is disposed between the constant densityregion and the central region. In some aspects, the density of thelattice is constant in the constant density region along a verticalaxis. In some embodiments, the network of pores extends from an exteriorsurface to the opening.

Additional aspects, features, and advantages of the present disclosurewill become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumbers indicate like features, and:

FIG. 1A is a perspective view of an interbody implant having a pluralityof visualization windows according to aspects of the present disclosure.

FIG. 1B is a perspective view of an interbody implant having a pluralityof visualization windows according to aspects of the present disclosure.

FIG. 2 is an exploded view of an interbody implant having a plurality ofvisualization windows according to aspects of the present disclosure.

FIG. 3A is a front elevation view of the interbody implant of FIGS. 1Aand 1B according to aspects of the present disclosure.

FIG. 3B is a rear elevation view of the interbody implant of FIGS. 1Aand 1B according to aspects of the present disclosure.

FIG. 3C is a side elevation view of the interbody implant of FIGS. 1Aand 1B according to aspects of the present disclosure.

FIG. 4A is a cross-sectional view of the interbody implant of FIGS. 1Aand 1B taken along line 4A-4A, according to aspects of the presentdisclosure.

FIG. 4B is a cross-sectional view of the interbody implant of FIGS. 1Aand 1B taken along line 4B-4B, according to aspects of the presentdisclosure.

FIG. 4C is a cross-sectional view of the interbody implant of FIGS. 1Aand 1B taken along line 4C-4C, according to aspects of the presentdisclosure.

FIG. 4D is a cross-sectional view of the interbody implant of FIGS. 1Aand 1B taken along line 4D-4D, according to aspects of the presentdisclosure.

FIG. 4E is a cross-sectional view of the interbody implant of FIGS. 1Aand 1B taken along line 4E-4E, according to aspects of the presentdisclosure.

FIG. 5 is a perspective view of a lumbar interbody implant having aplurality of visualization windows according to aspects of the presentdisclosure.

FIG. 6 is a perspective view of a lumbar interbody implant having aplurality of visualization windows according to aspects of the presentdisclosure.

FIG. 7A is a cross-sectional view of the interbody implant shown in FIG.5 taken along line 7A-7A, according to aspects of the presentdisclosure.

FIG. 7B is a cross-sectional view of the interbody implant shown in FIG.5 taken along line 7B-7B, according to aspects of the presentdisclosure.

FIG. 8A is an elevation view of a density or porosity gradient of avisualization window of an interbody implant according to aspects of thepresent disclosure.

FIG. 8B is a graphical view of a density or porosity gradient of avisualization window of an interbody implant according to aspects of thepresent disclosure.

FIG. 9 is a perspective view of an interbody implant having avisualization window being positioned between adjacent vertebrae of apatient's spine during a spinal fusion surgical procedure, according toaspects of the present disclosure.

FIG. 10 is a representation of a radiological image showing an interbodyimplant having a visualization window being positioned between adjacentvertebrae of a patient's spine, according to aspects of the presentdisclosure.

Although similar reference numbers may be used to refer to similarelements for convenience, it can be appreciated that each of the variousexample embodiments may be considered to be distinct variations.

DETAILED DESCRIPTION

Exemplary embodiments will now be described hereinafter with referenceto the accompanying figures, which form a part hereof, and whichillustrate examples by which the exemplary embodiments, and equivalentsthereof, may be practiced. As used in the disclosures and the appendedclaims, the terms “embodiment,” “example embodiment” and “exemplaryembodiment” do not necessarily refer to a single embodiment, although itmay, and various example embodiments, and equivalents thereof, may bereadily combined and interchanged, without departing from the scope orspirit of present embodiments. Furthermore, the terminology as usedherein is for the purpose of describing example embodiments only and isnot intended to be limitations of the embodiments. In this respect, asused herein, the term “plate” may refer to any substantially flatstructure or any other three-dimensional structure, and equivalentsthereof, including those structures having one or more portions that arenot substantially flat along one or more axis. Furthermore, as usedherein, the terms “opening,” “recess,” “aperture,” and equivalentsthereof, may include any hole, space, area, indentation, channel, slot,bore, and equivalents thereof, that is substantially round, oval,square, rectangular, hexagonal, and/or of any other shape, and/orcombinations thereof, and may be defined by a partial, substantial orcomplete surrounding of a material surface. Furthermore, as used herein,the term “in” may include “in” and “on,” and the terms “a,” “an” and“the” may include singular and plural references. Furthermore, as usedherein, the term “by” may also mean “from,” depending on the context.Furthermore, as used herein, the term “if” may also mean “when” or“upon,” depending on the context. Furthermore, as used herein, the words“and/or” may refer to and encompass any and all possible combinations ofone or more of the associated listed items.

FIGS. 1A and 1B are perspective views of an interbody implant device100. The implant device 100 is sized, shaped, and structurallyconfigured to be positioned between adjacent vertebrae in a human spine.In particular, the implant device 100 is configured to be positionedbetween cervical vertebrae. The implant 100 has a body including aplurality of lateral sides or faces defining and surrounding a cavity105. In particular, the implant 100 includes a front side 102, a lateralside 104, a rear side 106, and a lateral side 108. The implant 100 alsoincludes a top surface 112 and a bottom surface 114, each arrangedaround the cavity 105, which may also be referred to as an opening. Theimplant 100 includes apertured portions and solid portions, which willbe described further below with respect to FIG. 2. In some aspects, theapertured portions may be referred to as porous portions, where thepores include a network of holes or passages in the body of the implant100. The apertured portions may also be referred to as cavernousportions, cavitated portions, for example. The apertured portionscomprise a plurality of visualization windows 110 including one or moreapertures or pores extending into and/or through a thickness of eachside (e.g., 102, 104) of the implant 100. The implant 100 includesvisualization windows 110 on the front side 102, the lateral side 104,and the lateral side 108. The implant 100 also includes solidscaffolding portions or regions 120, which do not include the aperturespresent in the visualization windows 110. In the illustrated embodiment,the solid scaffolding portions 120 and the visualization windows 110form a singular or integral structure. For example, the implant 100 maybe manufactured using an additive manufacturing process, such as athree-dimensional (3D) printing process or metal sintering process inwhich the solid scaffold portions 120 and the visualization windows 110are formed together in a layered process. In other embodiments, thesolid scaffold portions 120 and the visualization windows 110 compriseseparate components joined together during manufacturing.

In other embodiments, the implant 100 may comprise separate componentscoupled or otherwise joined together during a manufacturing process. Forexample, the implant 100 may include a unitary frame structure similarto the scaffold portions 120, and the lattice structure, including thevisualization windows 110, may be inserted and joined to the framestructure during assembly. In some embodiments, the lattice structureand/or visualization windows 110 comprise a different material than thescaffold portions 120. For example, in some embodiments, the scaffoldportions 120 include a metallic material (e.g., titanium) and thevisualization windows 110 include a polymer (e.g., polyether etherketone (PEEK)). In other embodiments ,the visualization windows 110 areformed of a same type of material.

Referring to FIG. 1B, the rear side 106 of the implant 100 includes asolid surface, which may be referred to as non-porous or non-apertured.However, it will be understood that “porous” refers to the apertures orpassageways present in the visualization windows 110. The non-poroussurface of the rear side 106 may include micropores or nanopores in thesurface, which may manifest as a relatively rough surface to a humanobserver. The implant 100 includes two positioning holes 122 extendingthrough the rear side 106 and in communication with the cavity 105. Aswill be explained further below, the positioning holes 122 may beconfigured to receive a positioning tool (see 550, FIG. 9) for placementof the implant 100 between the vertebrae of the patient's spine.

The visualization windows 110 include a network or pores or aperturesextending through a thickness of the lateral sides of the implant 100.Thus, the density of the implant 100 in the regions of the visualizationwindows 110 is lower than the density of the implant 100 in the solidscaffold regions 120. Stated differently, the porosity of the implant100 in the regions of the visualization windows 110 is higher than theporosity of the implant 100 in the solid scaffold regions 120. In thisregard, the density of the implant 100, which has a porous structure,may be inversely related to the porosity of the implant 100. Theporosity of the implant may be described as the ratio of porous volumeto solid volume. The lower density of the visualization windows 110 mayfacilitated bone in-growth for better incorporation and long-termstability of the implant 100 within the patient's spine. Further, thelower density of the visualization windows 110 provides for improvedvisualization of bone in-growth within the cavity 105 of the implant100. In one aspect of the present disclosure, the density or porosity ofthe implant 100 in the visualization windows 110 may vary to provide forenhanced radiological visibility while providing sufficient strength tosupport the load of the patient's body. In this regard, the density ofthe apertured visualization windows 110 defined by interconnectedlattices may be relatively higher in a central region of thevisualization windows 110 than in an outer region of the visualizationwindows 110. In an inverse relationship, the lattices forming thewindows can be higher in an outer region than in a central region of thevisualization window. For the purposes of the present disclosure, theporous or apertured visualization windows may be described as havingporosity gradients, or density gradients. In this regard, because theporosity of the lattice structures, including the visualization windows110, varies, the density also varies inversely to the porosity.

In the illustrated embodiment, the apertured portion of the implant 100includes a gyroid architecture. The gyroid architecture includes aperiodic minimal surface defining a network or labyrinth ofinterconnected passageways. The geometric parameters of the gyroidstructure may be configured using one or more input parameters, whichmay adjust the size of the apertures (e.g., diameter), the spacingbetween apertures, or any other associated parameter. Accordingly, thedensity of each visualization window 110 may be configured by settingthe geometric parameters of the gyroid structure. As will be explainedfurther below, the density of each visualization window 110 may varyalong one or more axes. For example, in some embodiments, the density ofeach visualization window 110 may increase along a vertical axisextending from the bottom surface 114 of the implant 100 to the topsurface 112 of the implant 100 such that the density of thevisualization window 110 reaches a minimum at the center of thevisualization window, and reaches maxima at the upper and lower regionsof the visualization window 110. In other words, the porosity of eachvisualization window 110 may reach a maximum at the center of thevisualization window 110, and may reach minima at the upper and lowerregions of the visualization window 110. In some aspects, the density ofthe visualization windows 110 varies as the size of the apertures orpassageways of the apertured portion increases or decreases.

The apertures or passageways of the apertured portion of the implant 100may be interconnected to provide for improved bone in-growth. Althoughthe embodiment shown in FIGS. 1A and 1B has a gyroid architecture, itwill be understood that other types of apertured bodies are contemplatedby the present disclosure. For example, the implant 100 may includearrays of orthogonal apertures that intersect, or do not intersect. Inparticular, the implant 100 may include an apertured structure similarto that described in U.S. Pat. No. 9,693,874, issued Jul. 4, 2017, theentirety of which is hereby incorporated by reference. In otherembodiments, the porous or apertured portion of the implant 100 may beformed by pressing or melting together small metal beads or particles ofvarious sizes together to form the implant 100. The spaces or gapsbetween the metal particles may vary in size, such that the radiolucentsee between the visualization windows 110 increases toward the center ofeach visualization.

The embodiment of the implant 100 shown in FIGS. 1A and 1B may beconfigured for use between the cervical vertebrae of the patient.However, as will be explained further below, the present applicationalso contemplates interbody implants configured for use in other areasof the spine, such as between the lumbar vertebrae. The implant 100 maybe metallic, in some embodiments. For example, the implant 100 mayinclude titanium. In other embodiments, the implant 100 may be formed ofa polymer material, such as polyether ether ketone (PEEK). The polymermaterial may be radiopaque. For example, the polymer may be impregnatedwith particles of a radiopaque material, such as Barium Sulfate,Bismuth, Tungsten, and/or any other suitable metal or radiopaquematerial. In some embodiments, one or more of the components are formedof bone or other naturally occurring material with sufficient strengthfor use as an spinal implant. In other embodiments, one or morecomponents of the implant 100 may include a calcium-based substance.

FIG. 2 is an exploded view of the interbody implant 100 shown in FIGS.1A and 1B. In particular, FIG. 2 shows an apertured portion 130 and asolid scaffolding portion 140. Although shown separately, it will beunderstood that the apertured portion 130 and the solid scaffoldingportion 140 may form an integral single structure, as explained abovewith respect to FIGS. 1A and 1B. FIG. 2 illustrates, conceptually, thefunctional aspects of the apertured portion 130 and the solidscaffolding portion 140.

The apertured portion 130 varies in density or porosity along an axisextending from the bottom surface 114 and the top surface 112. Thedensity reaches a minimum at or near the center of the body along theaxis. The density decreases as the size of the apertures increases. Inother words, the porosity may be described as increasing as the size ofthe apertures of passageways increases. Stated differently, the densitydecreases as the interstitial surfaces defining the apertures decreasein thickness. As the interstitial surfaces forming the gyroid structureincrease in thickness for upper and lower regions of the aperturedportion 130, the density increases in the porosity decreases. Theinternal pore size (e.g., diameter) of the passageways in the aperturedportion 130 may vary from 150 μm to 700 μm. However, it will beunderstood that these values are merely exemplary and that the poresizes or diameters may have other values both greater or smaller.Further, although the size of the passageways may be described in termsof diameter in some instances, the passageways may not have circularcross-sections. In this regard, the term “diameter” may describe theaverage distance from one side or boundary of a passageway to anopposing side or boundary of the passageway.

The solid scaffolding portion 140 includes a plurality of scaffolds 120.Some of the scaffolds 120 may extend vertically from the bottom surface114 to top surface 112. Other scaffolds 120 extend horizontally at ornear the top surface 112 and the bottom surface 114. Solid scaffoldingportion 140 provides more strength and rigidity of the implant 100 tosupport the load of the spine. As shown in FIG. 2, the gyroid structuralapertures are not present in the solid scaffolding portion 140. However,it will be understood that the solid scaffolding portion may includemicropores or nano for formed by etching, sandblasting, or any othersuitable process to provide a rough surface. The roughened surface ofthe solid scaffolding portion 140 may provide for increased adhesion tothe bone and biological structures in the spine.

As will be explained further below, the apertured portion 130 mayinclude one or more regions of constant density or porosity along theaxis. The constant density portions may have a relatively higher density(or lower porosity) and may provide increased strength and rigidity ofthe apertured portion 130 around an exterior of the implant 100.Further, the gradient of the density of the apertured portion 130 maybalance the strength of the implant 100 with the increased radiolucencyor visibility through the visualization window 110. Accordingly, theimplant 100 allows for visualization through visualization windows 110while maintaining sufficient strength and rigidity to support the loadof the spine.

FIGS. 3A, 3B, and 3C are side elevation views of different sides orfaces of the implant 100. FIG. 3A is a side elevation view of the frontside 102, FIG. 3B is a side elevation view of the rear side 106 of theimplant 100, and FIG. 3C is a side elevation view of the lateral side108 of the implant 100. Referring to FIG. 3A, the implant 100 includeson the front side 102, a first visualization window 110 a disposed on acentral region of the front side 102 and corner visualization windows110 b and 110 c on the outer edges or corners of the front side 102. Thefront side 102 also includes scaffold portion 120 between the firstvisualization window 110 a and the corner visualization windows 110 b,110 c. The scaffold portions 120 also extend horizontally along thebottom surface 114 and the top surface 112 of the implant 100. Theporosity of all three visualization windows 110 a, 110 b, and 110 cincreases toward the center of each visualization window 110.

As shown in FIG. 3B, the rear side 106 of the implant 100 includes asolid surface and two positioning holes 122. The solid surface of therear side 106 may extend through an entire thickness of the rear side106. The solid portion of the rear side 106 may provide for increasedstrength for the positioning holes 122. The positioning holes 122 areconfigured to receive a positioning tool for placement of the implant100 in between adjacent vertebrae of the patient's spine. In someaspects, a significant amount of force and impact may be applied to theimplant 100 via the positioning holes 122. Accordingly, the solidportion of the rear side 106 provides increased strength against theforce and impact during placement of the implant 100 patient's spine. Inthe illustrated embodiment, the rear side 102 may be largest side. Theshape of the implant 100 and the proportionality of the front side oneof to the rear side 106 lateral sides 104, 108 may be configured basedon the curvature of the patient's spine (e.g., lordosis) at the locationof the implant 100. In this regard, the lateral side 108 is shown ashaving a trapezoidal, or wedge shape. The wedge-shaped of the lateralside 108 (and the corresponding lateral side 14) may coincide with thecurvature of the patient's spine. The lateral side 108 shown in FIG. 3Chas a single visualization window 110 d and solid scaffold portions 120on each side of the visualization window 110 d.

FIGS. 4A, 4B, 4C, and 4D are cross-sectional views of the implant 100shown in FIG. 1A taken along lines 4A-4A, 4B-4B, 4C-4C, and 4D-4D,respectively. FIGS. 4A and 4B are cross-sectional views of the implant100 taken along parallel planes in the front side 102. The cross-sectionof FIG. 4A, which is taken along line 4A-4A, is closer to the outersurface of the first side 102. Accordingly, FIG. 4A shows the solidscaffold portions 120 separating the first visualization window 110 afrom the corner visualization windows 110 b and 110 c. By contrast, FIG.4B is taken along a plane closer to the cavity 105 of the implant 100such that the solid scaffold portions 120 separating the firstvisualization window 110 a from the corner visualization windows 110 b,110 c are not present. In this regard, it will be understood that thesolid scaffold portions 120 extending vertically along the implant 100are present in only a portion of the thickness of the first side 102.Accordingly, the porous structure of the implant 100 around the cavity105 allows for increased bone in-growth and adhesion to the vertebrae.

FIG. 4B shows solid scaffold portions 120 at the left and right edges orcorners of the implant 100. FIG. 4B also shows a portion of the centralcavity 105. The porous structure of the implant 100 around the cavity105 may form an interconnected network or lattice of passagewaysarranged in a periodic fashion, in which the passageways curve throughthe sidewalls of the implant 100 and intersect with one another.

Referring still to FIGS. 4A and 4B, the density or porosity gradient ofthe first side 102 is constant throughout the thickness of the firstside 102. Accordingly, in the cross-sections of both FIG. 4A and FIG.4B, the porosity increases toward the center of the visualizationwindows 110 and decreases toward the upper and lower portions of thevisualization window 110. In some aspects, the upper and lower portionsof the visualization window 110 may be referred to as outer portions orouter regions of the visualization window 110. The porosity may varylinearly or non-linearly. In the illustrated embodiment, the porosityvaries in an ordered fashion according to a geometrical functiondescribing a gyroid.

FIG. 4C shows a cross-sectional view of the implant 100 taken along line4C-4C, which extends from the front side 102 to the rear side 106 andthrough the cavity 105. The apertures or passageways of thevisualization window 110 on the front side 102 extend through an entirethickness of the front side 102, and are in communication with thecavity 105. By contrast, the rear side 106 includes a solid portionextending through an entire thickness of the rear side 106.

FIG. 4D shows a cross-sectional view of the implant 100 taken along line4D-4D, which extends through the rear side 106. The solid portion 126 ispresent throughout a thickness of the rear side 106 and surrounds ordefines the positioning holes 122. The solid portion 126 extends aroundthe corners 128 adjacent to the lateral sides 104, 108.

FIG. 4E shows a cross-sectional view of the implant 100 taken along line4E-4E. The cross-section shown in FIG. 4E shows a middle section of theimplant 100, including the cavity 105. In some aspects, thecross-sectional view of FIG. 4E may be associated with a central regionof the visualization windows 110 which have a heightened porosity andlower density. Because the cross-section of FIG. 4E is taken along aplane of constant height through the implant 100, the porosity may beconstant across this plane. The solid scaffold portions 120 havewedge-shaped cross-sections such that the widest portions of the solidscaffold portions 120 are at the outer surfaces of the implant 100, andthe solid scaffold portions 120 become increasingly narrow as theyprotrude inward toward the cavity 105. An interior surface of theimplant 100 surrounding the cavity 105 may include an entirely or mostlyapertured or porous surface. In the visualization windows 110 theapertures or passageways may be present in an entire thickness 118 ofthe implant 100.

Referring generally to FIGS. 1A-4E, one or more of the features of theimplant 100 may be modified, adjusted, removed, or otherwise changedwithout departing from the scope of the present disclosure. For example,in some embodiments, one or more of the sides (102, 108) of the implantmay include more than one visualization window 110 divided or defined byone or more additional solid scaffold portions 120. In some embodiments,the front side 102 and/or one of the lateral sides 104, 108 may notinclude a visualization window 110, and may be solid. In someembodiments, the front side 102 and/or one of the lateral sides 104, 108may include a visualization having a constant porosity, such that onlyone of the visualization windows has a functional gradient for improvedradiotransparency. In some embodiments, the rounded corners of theimplant 100 are solid, and do not include visualization windows 110. Insome embodiments, at least a portion of the rear side 106 includes aporous portion, and may include a visualization window 110. In someembodiments, the porosity of the visualization windows 110 variesthrough the thickness of the sidewall toward the cavity 105. Forexample, the porosity may increase from an exterior surface of theimplant 100 to an interior surface of the implant 100, wherein theinterior surface surrounds and defines the cavity 105.

FIGS. 5 and 6 are perspective views of lumbar interbody implants 200,300, according to aspects of the present disclosure. Referring to theimplant 200 shown in FIG. 5, the implant 200 includes a plurality ofvisualization windows 210 and a plurality of scaffold portions 220,similar to the implant 100 shown in FIGS. 1A-4E. The implant 200includes a plurality of lateral sides or surfaces, including a lateralside 204, a rear side 206 and a lateral side 208, each arranged around acavity 205. The implant 200 includes apertured portions and solidportions. The apertured portions may be referred to as porous portions,where the pores include a network of holes or passages in the body ofthe implant 200. The apertured portions may also be referred to ascavernous portions, cavitated portions, for example. The implant 200also includes a nose portion 230 including one or more solid surfaceshaving an angled or conical shape. The visualization windows 210 and thesolid scaffold portions 220 and the nose portion 230 define and surrounda cavity or opening 205. The opening 205 may extend from a top surface212 to a bottom surface 214. In some embodiments, the opening 205extends from the top surface 212 to an intermediate portion such thatthe opening 205 does not extend through an entire height of the implant200. Similar to the implant 100 shown in FIGS. 1A and 1B, the rear side206 of the implant 200 includes a positioning hole 222. The positioninghole 222 is configured to receive a positioning tool for placement ofthe implant 200 and between the vertebrae of the patient's spine. Therear side 206 comprises a solid surface surrounding and defining thepositioning hole 222. The solid surface or portion of the rear side 206may extend an entire thickness of the rear side 206, or a portion of thethickness of the rear side 206.

Referring still to FIG. 5, a solid scaffold portion 220 is disposed in acentral region of the lateral side 204 between two visualization windows210 a, 210 b. An identical or similar scaffold portion 220 may bepresent on the lateral side 208. In some aspects, the additional solidscaffold portions 220 shown in the embodiment of FIG. 5 provideadditional support and strength for the implant 200, which is longerthan the implant 100 shown in FIGS. 1A and 1B. Further, it will beunderstood that the implant 200 shown in FIG. 5 may experience largerforces than the implant 100 shown in FIGS. 1A and 1B, since the implant200 is used in between the lumbar vertebrae and not the cervicalvertebrae. The visualization windows 210 have varying densities along atleast one direction or axis. In this regard, the visualization windows210 have a density gradient or porosity gradient along a height of theimplant 200 such that density is the lowest at a middle height of theimplant 200.

The nose portion 230 facilitates insertion of the implant 200 betweenthe lumbar vertebrae. For example, when advancing the nose portion 230of the implant 200 in a space between adjacent vertebrae of the spine,the vertebrae may separate to provide space for the implant 200. Similarto the implant 100 shown in FIGS. 1A and 1B, the implant 200 may includea wedge shape such that the rear side 206 is smaller or shorter in thedirection compared to the height at the base of the nose portion 230.Accordingly, the implant includes an inclined or wedged shape profileconfigured to conform to the curvature of the spine (lordosis). In someaspects the implant 200 may be used in a pair such that two implants 200are positioned next to each other between the lumbar vertebrae.

Referring to FIG. 6, and implant 300 includes a nose portion 330visualization windows 310 solid scaffold portions 320 a rear side 306lateral sides 304, 308, each arranged around a cavity 305. The implant300 includes apertured portions and solid portions. In some aspects, theapertured portions may be referred to as porous portions, where thepores include a network of holes or passages in the body of the implant300. The apertured portions may also be referred to as cavernousportions, cavitated portions, for example. The implant 300 has a curvedshape for use in the lumbar vertebrae in a different configuration thanthe implant 200. For example, the implant 300 may be inserted betweenthe lumbar vertebrae in a sideways direction (e.g., transpsoasapproach). In another example, the implant 300 may included in a pair ofsimilar implants which extend in parallel between adjacent vertebrae inthe spine. In some aspects, the size of the lateral side 308 may bedifferent from the size of the lateral side 304, to account for thecurvature of the spine.

FIGS. 7A and 7B are cross-sectional views of the implant 200 shown inFIG. 5 taken along the lines 7A-7A and 7B-7B, respectively. Referring toFIG. 7A, the implant 200 includes solid scaffold portions 220,visualization windows 210, and a solid nose portion 230. The solidscaffold portions 220 form a structural frame for the implant 200 toprovide strength and resist deformation under pressure in the spine. Thesolid nose portion 230 may be rigid and durable to facilitatepositioning of the implant 200 between vertebrae. In this regard, thenose portion 230 of the implant 200 may be inserted between adjacentvertebrae, and pushed or tapped into place using a positioning toolcoupled to the implant by the positioning hole 222. In the illustratedembodiment, the positioning hole 222 is threaded and configured tocouple to a corresponding threaded positioning tool.

The visualization windows 210 are shown on one side of the implant 200,with a vertically-extending scaffold portion 220 positioned between thevisualization windows 210. The visualization windows 210 are at leastpartially radiolucent or radiotransparent such that the cavity of theimplant 200 can be monitored and inspected using x-ray imaging. In oneaspect, the cavity of the implant is filled with bone growth promotingmaterial and the visualization window permits radiograph inspection ofthe progress of bone growth over a period of time after implantation,such as days, weeks, months or years. The visualization windows 210 havea gradient of porosity or radiotransparency that increases toward acenter of the visualization windows 210 along at least a verticaldirection from the top surface 212 to the bottom surface 214, which maybe referred to as the vertical axis. In some embodiments, the gradientof porosity or radiotransparency of the visualization windows 210 alsovaries along the horizontal axis. The visualization windows 210 mayinclude constant porosity or constant radiotransparency regions near thetop and/or bottom of the visualization windows 210, in some embodiments.

FIG. 7B is a cross sectional view of the implant 200 taken along line7B-7B, which shows a largely solid region of the body of the implant 200at the rear solid scaffold portion 220 near the positioning hole 222.The solid scaffold portion 220 extends around the corners of the implant200, and porous portions 232 are present on a top side and a bottom sideof the implant 200. The lateral sides include recesses 234, and theexterior surfaces of the visualization windows 210 can be seen beyondthe recesses 234. The cavity 205 extends through the implant 200 fromthe top surface 212 to the bottom surface 214. The porous portions 232have a lower porosity than the visualization windows 210, which arecloser to a center of the implant with respect to the vertical axis.Accordingly, the porous portions 232 may allow for bone growth into theporous structure, but provide less radiotransparency than thevisualization windows 210.

Although the implants 200 and 300 shown in FIGS. 5-7B have differentshapes, sizes, and structural configurations than the implant 100 shownin FIGS. 1A-4E, each of the implants 100, 200, 300 allow forradiological imaging through the visualization windows 110, 210, 310through at least one side of the implant, while maintaining sufficientstrength to support the load applied to each device when implanted inthe spine. The implants 100, 200, 300 each have reinforcing structures,referred to in some embodiments as scaffolding portions, to providestrength and rigidity around the edges or perimeters of thevisualization windows 110, 210, 310, which are radiotransparent in atleast a central region to allow for monitoring of bone in-growth in thecavities of each implant. The scaffolding portions provide a skeleton,with the visualization windows occupying the space between portions ofthe skeleton. The particular gradient used for each type of implant, thesize of the scaffolding portions, the overall dimensions of the implant,the thickness of the sidewalls, and other geometric and structuralparameters may be selected based on the application of the implant(e.g., lumbar, cervical, etc.).

FIGS. 8A and 8B illustrate the gradient or density profile of avisualization window 400, according to some embodiments of the presentdisclosure. The visualization window 400 may be one of the visualizationwindows 110, 210, or 310, or may otherwise be applied to the implants100, 200, and/or 300. FIG. 8A is an elevation view of a density orporosity gradient of a visualization window 400 of an interbody implant.FIG. 8B is a graphical view of the density or porosity gradient of thevisualization window. As similarly described above, the visualizationwindow 400 includes a lattice or network of structures definingchannels, tunnels, or pores in a sidewall of the implant. On the surfaceshown in the illustrated embodiment, the porosity or density isassociated with a size (e.g., diameter) of the openings or apertures404, which are periodically arranged across the visualization window400, with each row of apertures offset from a neighboring row by halfthe distance between neighboring apertures in a given row. Stateddifferently, the density is associated with a proportion of solidportions 402 to the apertures 404. The lattice or network of solidstructures, surfaces, and apertures forms a gyroid structure, such thatthe apertures 404 are interconnected. The visualization window 400varies in porosity along a vertical axis.

The visualization window 400 includes constant density regions 416 atthe top and bottom portions of the visualization window 400, whichextend from the top surface 412 and bottom surface 414. In the constantdensity regions 416, the size of the apertures 404 does not increase ordecrease. Accordingly the density of the constant density regions 416 isrelatively high. For example, referring to FIG. 8B, the density of theconstant density regions 416 is approximately 50%, and is constant orflat throughout the constant density regions 416.

The visualization window 400 also includes outer regions 422, 424, and acentral region 420. In FIG. 8A, the outer regions 422, 424, and centralregion 420 may be defined arbitrarily based on the density of thelattice at various locations along the vertical axis of the window 400.In this regard, in the embodiment of FIGS. 8A and 8B, the porosityvaries linearly from outer boundaries of the outer regions 422, 424 to acenter of the central region 420. The porosity reaches a maximum of 80%at the center of the central region 420. Thus, the outer regions 422,424 may be defined as having relatively lower porosity closer to 50%,and the central region may be defined as having relatively higherporosity closer to 80%. Accordingly, the central region 420 is moreradiotransparent than the outer regions 422, 424. In some embodiments,the central region 420 may be sufficiently radiotransparent to allow forx-ray imaging through an entire thickness of the visualization window400 into an interior cavity of the implant. In some embodiments, thecentral region 420 may be sufficiently radiotransparent to allow x-rayimaging of material beyond the central cavity, and even outside of theimplant on an opposing side. For example, the central region 420 may besufficiently radiotransparent to allow x-ray imaging of material withinan opposing sidewall and/or an opposing visualization window on theother side of the device.

The porosity, as a percentage, may be determined by dividing porousvolume with the overall volume. In some aspects, the porosity isinversely related with the lattice density. For example the porosity maybe inversely proportional to lattice density. When using an orderedporous structure as shown in FIG. 8A, the porosity may be based on orotherwise associated with the diameter of the apertures 404 at a givenvertical position. For example, at the center of the visualizationwindow 400 with a porosity of 80%, the diameter of the apertures 404 atthis location may range from 550 μm to 750 μm. In the constant densityregions with a porosity of 50%, the diameter of the apertures may rangefrom 150 μm to 550 μm. In some embodiments, the porosity may becontrolled or modified based on a spacing of the apertures 404 in thelattice structure. Further, it will be understood that the 50% and 80%porosity values are exemplary, and do not limit the scope of the presentdisclosure. For example, in some embodiments, the porosity of theconstant density regions may be between 30% and 55%. In someembodiments, the maximum porosity at the central region 420 of thevisualization window 400 may be between 70% and 90%.

The porosity of the visualization window 400 at a given position alongthe vertical axis is based on the size of the apertures 404. The size ofthe apertures 404, and therefore the porosity, linearly increases in theouter regions 422, 424 and the central region 420 as the distance fromthe center of the central region 420 decreases. For example, a diameter426 of an aperture 404 at a first position in the outer region 422 issmaller than a diameter 428 of an aperture 404 near the center of thecentral region 420. Described in another, the proportion of the solidportions 402 to apertures 404 decreases from the outer regions 422, 424to the center of the central region 420.

The size of the apertures may vary linearly in the lattice or network ofthe window 400 based on a function defining a shape of the latticestructure. For example, the size of the apertures 404 at a givenvertical position may be determined or based on one or more coefficientsof a gyroid equation used to determine the shape of the latticestructure. In other embodiments, the porosity may be based on a spacingof the apertures 404 from the neighboring apertures, and/or a number ofapertures 404 for each unit of surface area or volume (e.g., cm², cm³).

In some embodiments, the porosity or density of the visualization window400 varies along the horizontal axis in addition to the vertical axis.For example, in some embodiments, the visualization window 400 includeslateral outer regions having a lower porosity than the central region.The density may vary linearly or non-linearly along the horizontal axis.The density may also vary though a thickness of the visualization window400, in some embodiments. Further, in some embodiments, thevisualizations windows described herein may comprise rectangular ornon-rectangular shapes, including circular shapes, elliptical shapes,hexagonal shapes or any other suitable shape. The arrangement of theapertures 404 or pores may be ordered or random. In other embodiments,the density gradient may be non-linear. For example, the densitygradient may follow a gaussian curve, a quadratic curve, a linearstepped function, or any other suitable type of profile.

FIG. 9 is a perspective view of an interbody implant 500 having avisualization window 510 being positioned between adjacent vertebrae 52,54 of a patient's spine, according to aspects of the present disclosure.The implant 500 is positioned using an insertion tool 550. The insertiontool may be coupled to the device via one or more positioning holes(e.g., 122, FIG. 3B) on a side of the implant 500. The visualizationwindow 510 is positioned on a lateral side of the implant 500, such thatan interior of the implant 500 may be viewed through the visualizationwindow 510 by obtaining an x-ray image along a coronal plane of thepatient. For example, the implant 500 may include an internal cavity oropening. The implant 500 is configured to bind with the vertebrae 52, 54using a bone graft material. The bone in-growth within the cavity of theimplant 500 may be observed by obtaining x-ray images through thevisualization window. The radiotransparency of at least a portion of thevisualization window 510 may be sufficient to allow for monitoring thebone in-growth in the cavity, and within the cavernous structure of thevisualization window.

FIG. 10 is a representation of a radiological image of an interbodyimplant 600 having a visualization window 610 and being positionedbetween adjacent vertebrae 52, 54 of a patient's spine, according toaspects of the present disclosure. The visualization window 610 includesa functional gradient in which a porosity or density increases along atleast one axis, according to an embodiment of the present disclosure.The implant 600 further includes one or more solid scaffold portions 620around a perimeter of the lateral sides of the implant 600. The scaffoldportions 620 provide for added strength to allow the implant 600 toretain its shape, even with relatively lower density and porous portionsin the visualization window 610.

As mentioned above, with the implant 600 in place, the visualizationwindow 610 allows the physician to observe the progress of bonein-growth in the implant 600. Further, the scaffold portions 620 and thefunctional gradient of the visualization window 610 allow for enhancedradiotransparency while maintaining structural strength and integrityunder the force of the spine.

As mentioned above, the implants described herein (e.g., 100, 200, 300,500, 600) may be manufactured using an additive manufacturing process.For example, the implants may be formed by a 3D printing process, or bya metal sintering process. In some embodiments, an implant is formed bydepositing a layer of metallic (and/or polymer) power on a substrate,and laser sintering the portions of the layer that will be incorporatedinto the device. The geometry of the layers may be determined based on apre-defined porous geometric structure (e.g., gyroid), where individualslices of the geometric and extracted and rendered according to theoperating parameters of the metal sintering device. This process can berepeated layer-by-layer until the implant is formed. The un-sinteredpower can be removed from the structure, leaving a unitary or monolithicimplant which has a functional gradient including the lattice structuresdescribed herein.

According to one embodiment, an implant may be formed by depositing orsintering a plurality of regions having different densities or densityranges. For example, a first region having a first density or firstdensity range may be deposited, printed, sintered, or otherwise formed.The first region may include a lattice structure having a network ofinterconnected passageways or pores having a first pore size. Forexample, the first region may correspond to the constant density regions416 shown in FIGS. 8A and 8B. Accordingly, in some embodiments, thefirst region comprises a first lattice density along at least one axis,and in particular, a vertical axis.

In a following step, a second region having a second density or seconddensity range may be deposited, printed, sintered, or otherwise formedon top of the first region. The second density or second density rangemay have a lower density than the first region. For example, the secondregion may correspond to the outer region 424 shown in FIG. 8A.Accordingly, the second region may have a gradient of lattice densitythat varies along at least the vertical axis.

In a following step, a third region having a third density or thirddensity region may be deposited, printed, sintered, or otherwise formedon top of the second region. The third density or third density rangemay have a lower density than the second region. For example, the thirdregion may correspond to the central region 420 shown in FIG. 8A.Accordingly, the third region may have a gradient of lattice densitythat varies along at least the vertical axis. In particular, the thirdregion may vary such that the density reaches a minimum at or near acenter of the third region along the vertical axis.

In some embodiments, the method for manufacturing the implant furtherincludes depositing a fourth region and a fifth region on top of thethird region. For example, the fourth region may correspond to the outerregion 422, and the fifth region may correspond to the upper constantdensity region 416 shown in FIG. 8A. Accordingly, the density profile ofthe implant may be symmetrical about a horizontal plane extendingthrough the center of the implant.

In some embodiments, depositing each of the first, second, and thirdregions includes depositing a plurality of layers that varyincrementally according to a geometric function, such as a gyroidfunction. The parameters of the gyroid function may determine the poresize of the lattice structure, and therefore the lattice density, ateach individual layer or slice.

While various embodiments in accordance with the disclosed principleshave been described above, it should be understood that they have beenpresented by way of example only, and are not limiting. Thus, thebreadth and scope of the invention(s) should not be limited by any ofthe above-described exemplary embodiments, but should be defined only inaccordance with the claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 C.F.R. 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically, a description of a technology in the “Background” is notto be construed as an admission that technology is prior art to anyinvention(s) in this disclosure. Furthermore, any reference in thisdisclosure to “invention” in the singular should not be used to arguethat there is only a single point of novelty in this disclosure.Multiple inventions may be set forth according to the limitations of themultiple claims issuing from this disclosure, and such claimsaccordingly define the invention(s), and their equivalents, that areprotected thereby. In all instances, the scope of such claims shall beconsidered on their own merits in light of this disclosure, but shouldnot be constrained by the headings herein.

What is claimed is:
 1. An interbody device configured for insertionbetween adjacent vertebrae, the interbody device comprising: a bodycomprising and exterior surface and an interior surface defining acavity, wherein the body comprises a visualization window extendingbetween the exterior surface and the interior surface, wherein thevisualization window comprises a lattice of radiopaque structures,wherein a density of the lattice in a central region of thevisualization window is less than in the density of the lattice in anouter region of the visualization window such that the visualizationwindow is radiolucent through the central region.
 2. The interbodydevice of claim 1, wherein the body comprises a top side and a bottomside, and wherein the lattice comprises a first density gradient along afirst direction extending between the top side and the bottom side. 3.The interbody device of claim 2, wherein the lattice comprises a seconddensity gradient along a second direction transverse to the firstdirection.
 4. The interbody device of claim 2, wherein the first densitygradient linearly decreases from the outer region of the visualizationwindow to the central region of the visualization window.
 5. Theinterbody device of claim 1, wherein the density of the lattice of theouter region of the visualization window is associated with a porosityof 40%-60%, and wherein the density of the lattice of the central regionof the visualization window is associated with a porosity of 70%-90%. 6.The interbody device of claim 1, wherein the visualization windowcomprises a first constant density region in the outer region.
 7. Theinterbody device of claim 6, wherein the first constant density regionextends from a bottom edge of the body to a first intermediate regionbetween the bottom edge and the central region of the visualizationwindow.
 8. The interbody device of claim 1, wherein the lattice ofradiopaque structures comprises a gyroid architecture.
 9. The interbodydevice of claim 8, wherein the density of the lattice is based ongeometric parameters of the gyroid architecture.
 10. The interbodydevice of claim 1, wherein the body comprises a plurality of solidscaffold regions, wherein the visualization window is disposed betweentwo solid scaffold regions.
 11. The interbody device of claim 10,wherein the body comprises a sidewall extending from the exteriorsurface to the interior surface, and wherein the plurality of solidscaffold regions occupy only a portion of a thickness of the sidewall.12. The interbody device of claim 1, further comprising a solid wall ona lateral side of the body, wherein the body defines one or morepassages extending through the solid wall on the lateral side of thebody.
 13. The interbody device of claim 1, wherein the body comprises asidewall extending from the exterior surface to the interior surface,and wherein the lattice of the visualization window occupies an entirethickness of the sidewall.
 14. The interbody device of claim 1, whereinthe lattice of radiopaque structures comprises a metal.
 15. Theinterbody device of claim 14, wherein the lattice of radiopaquestructures comprises titanium.
 16. An intervertebral spacer configuredto be placed between two adjacent vertebrae, the intervertebral spacercomprising: a solid portion comprising a plurality of solid scaffoldregions, wherein the solid scaffold regions are disposed around anopening; and a visualization window disposed between two or more of thesolid scaffold regions and around the opening, wherein the visualizationwindow comprises a lattice of radiopaque surfaces defining a network ofpores, wherein a density of the lattice is greater in an outer region ofthe visualization window than in a central region of the visualizationwindow such that the visualization window is radiolucent in the centralregion.
 17. The intervertebral spacer of claim 16, wherein the solidportion and the visualization window form a unitary body.
 18. Theintervertebral spacer of claim 16, wherein the lattice varies linearlyin density from the outer region to a center of the central region alonga vertical axis.
 19. The intervertebral spacer of claim 16, wherein thevisualization window further comprises a constant density regionpositioned above the outer region such that the outer region is disposedbetween the constant density region and the central region, wherein thedensity of the lattice is constant in the constant density region alonga vertical axis.
 20. The intervertebral spacer of claim 16, wherein thenetwork of pores extends from an exterior surface to the opening.